Coupling circuits



July 4, 1939. T. P. KINN COUPLING CIRCUITS Filed Sept. 16, 1937 kze fizcflfl'ed 191C l ower Sup 794a WIT NESSES:

INVENTOR Theo dare A A inn ATTORNEY Patented July 4, 1939 UNITED STATES PATENT OFFICE COUPLING CIRCUITS Pennsylvania Application September 16, 1937, Serial No. 164,163

Claims.

This invention relates to high-frequency coupling circuits tunable over an extended frequency range, and more particularly to circuits adapted for interconnecting successive stages of a 5 thermionic amplifier in a radio system.

In radio receivers tunable over a wide frequency range by means of a variable condenser resonating a fixed inductance, it is customary to employ plug-in coils, or a bank of fixed inif) ductances which may be selected by multi-tap switches, in order to extend the tuning range. The disadvantage of systems of this type resides in the complicated structure of the receiver. Furthermore by the use of the same variable condenser for several frequency ranges, the efficiency of voltage transfer becomes progressively lowered as the ratio of inductance reactance to capacitive reactance decreases.

It is the primary object of this invention to extend the tuning range of radio systems by providing a coupling circuit tunable over a Wide range without necessitating the use of plug-in coils or complicated switching mechanisms.

A particular feature of this invention is that 5 by the novel coupling circuit a high eificiency of signal transfer is obtained throughout the tuning range, by maintaining the ratio of inductive to capacitive reactance uniform over the tuning range.

30 Radio receivers having extended tuning ranges usually include the very high-frequency, or generally known as the shortwave portion of the broadcast band. Within this band the inductances must necessarily be made small in 35 order to provide the proper tuned circuit with the variable condenser used,which is the only tuning means for all the frequency ranges covered by the receiver. Consequently, at the higher frequency end of the range the L/C ratio previously mentioned will be the lowest, to the detriment of voltage transfer. Attempts have been made to design variable condensers having several minimum capacity ranges or to employ several interconnected variable condensers of dif- 45 ferent sizes together with selective means for including the proper condenser with each frequency range. In both cases, mechanical as well as electrical difficulties discourage the constructor and from a production standpoint greatly in- .50 crease the cost of a receiver.

Another feature of this invention is the use of an inductance having progressive varying electrical characteristics as the tuning means instead of a variable condenser. Inductances of the above type are generally known as variometers and consist of two coils whose relative position to each other may be varied in such manner that by the aiding or cancelling effects of the current in both coils, depending upon their relative position, the effective inductance may be 5 progressively varied within wide limits. It is to be understood that the invention is not to be restricted to the type of variometer described and any variable inductance may be used as well. 10

Additional objects and advantages will become apparent from the following description of the invention, defined in particularity by the appended claims, and taken in connection with the accompanying drawing, in which identical referl5 ence characters denote similar elements in the figures.

Referring to the drawing:

Figure 1 schematically shows a portion of a receiving system, wherein the coupling circuit of the present invention is used to interconnect successive amplifying stages; and,

Fig. 2 shows the elementary coupling circuit.

The portion of the receiving system shown in Fig. 1 comprises two stages of high-frequency amplification and a detector stage of the diode rectifier type. Other components generally required for a complete system, such as the audio amplifier and loud-speaker are not shown here in order to simplify the drawing by omitting elements ancillary to the system and having no direct bearing on the invention herein described.

The tunable high-frequency stages of the receiving system include the amplifying vacuum tubes I and 2 preferably of the screen grid type. Other types of tubes may as well be used with slight modification of the circuit in accordance with the operating characteristics of the particular tubes chosen.

Referring to the first stage of the amplifier, 40 the input circuit includes a primary inductance I, connected between antenna and ground, and forms part of the primary or input circuit of the coupling system. The secondary circuit thereof is the grid circuit of the first amplifying tube 1, and may be. divided into two parts: the first, comprising the fixed. secondary inductance 8, which is electromagnetically coupled to the primary inductance I, and the second, the variable inductance 9, or variometer. The latter as previously stated, consists of two serially connected coils, H, and it, which as the arrow schematically indicates may be physically so displaced as to vary the inductive relationship between coils" H and 10. One terminal of the variometer 8 56 voltage divider 29.

connects to the grid 4 of the tube I and the other to the inductance 8. The variometer 9 and the inductance 8, thereby are in series between the grid 4 and the grid return circuit. The latter comprising resistors I2, I3, I5, and the load resistance I of the diode 3 returns to ground through the cathode I6. The grid return circuit just described is of the conventional diode A. V. C. type and supplies negative bias voltage to the grids 4 and I of the amplifying tubes, I and 2 respectively, automatically in proportion with the strength of the received signal. Circuits of this type are well known in the art and need not be further described here, inasmuch as the present invention is not directly related to such circuits which are here illustrated merely to show the adaptability of the coupling system to modern types of receivers. Condensers I1 and II, bypassing the grid circuit, provide low impedance paths for high-frequency currents.

Continuing the description of the grid circuit, a single pole three position switch 20 has its moving arm 2| connected to the grid 4 of the tube I. The contact 24, at one extreme position of the switch arm, connects to the junction point of the coil 8 and winding II of the variometer 9. The contact 22 at the other extreme position, connects to one terminal of the condenser 23, the other terminal of which connects to ground. The arm 2I in its midway position makes no electrical connection.

v The cathode 6 of tube I is grounded through the resistor I8, by-passed by'the condenser I l. The screen electrode 5, by-passed by condenser 25,returns to a suitable tap on the voltage divider of the power supply which is sechematically shown, comprises filter reactor 26, filter condensers 2'! and 28 and the voltage divider 29.

The second stage of the amplifier system is identical in circuit arrangement with the first stage just described excepting that the input coil in the primary circuit of the coupling system is connected to the anode 38 of the preceding tube I and to the anode potential terminal of the The by-pass condenser 3| provides a low impedance path for the amplified high-frequency currents in the anode circuits. The grid input circuit, which is in fact the secondary circuit of the coupling system, is the 1 same as that of the first stage, and identical elements therein are. designated with identical reference character bearing primary indices, including the elements of the second amplifier tube 2. The output of the second tube may be coupled to the detector tube 3 in any suitable manner, for instance as shown by the untuned transformer 32. The modulation component of the rectified signal may be taken off by the condenser 33 and. conducted to a utilization circuit, for instance a suitable-number of audio frequency amplifying stages, followed by a loud-speaker.

- The description of Fig. 1 illustrates the coupling system of the present invention functioning as a signal transfer means between amplifying states of a receiving circuit- In Fig. 2, the elementary coupling system is shown which may advantageously be employed in any circuit as a tunable coupling device as long as the input andoutput impedance are properly calculated to match the preceding and succeeding network, respectively. The elements of the coupling circuit are identical with those described in connection with Fig. 1, except that the variometer 9 has no mechanical coordination with the switch 2! This feature is optional and particularly advantageous in re- 2,165,058 U c A ceiving arrangements where several elementary coupling circuits are interconnected for uni-control.

In describing the operation of the coupling system a better understanding may be had by first referring to Fig. 2. The primary coil I, may be connected to any suitable source of radio frequency energy encompassing a wide band of frequencies, especially including the very high frequencies of the shortwave spectrum at one end, and terminating, let us say, at the broadcast band between 15,000 to 550 K. C. Starting with the short-wave band, by means of the switch 20, having its'ar'm 2| contacting with terminal 24, the secondary tuning inductance shown here as the variometer 9 is efiectively short-circuited, and, therefore only the relatively small inductance 8 forms the secondary circuit. In order to obtain even at the highest frequency end of the range, a high ratio of inductive. vs. capacitive reactance, the inductance 8 forms a resonant circuit merely by its inherent distributed capacity together with other inherent capacities of conductors or with the input capacity of vacuum tubes if the coupling system is employed between amplifying stages as shown in Fig. 1. In the second or midposition of the switch 20, the short circuit is removed from the variometer 9, which now is in series with the inductance 8. The total effective inductance with the distributed capacity of the variometer and other associated capacities formerly stated, form the resonant circuit in the second range which is tunable by varying the coupling between the coils I0 and II of the variometer 9. The secondary circuit is predominantly inductive at the same time allowing tuning over a wide range. In the third position of the switch 28, the arm 2| is contacting terminal 22 whereby the fixed condenser I8 is shunted across the series combination of the inductances 3 and the variometer 9. By adding capacitive reactance to the circuit in the form of a condenser the tuning range is further extended al though at the lower frequencies to be covered in the third position the ratio of inductive to capacitive reactance is still maintained substantially coupling circuit, as shown by the dotted lines in Fig. 1.

The type of mechanical intercoupling is not shown here in detail, for any of the well known types may be used, such for instance, as employed in receivers at the present time to interconnect several tuning condensers. It is only necessary that the mechanical system be so designed as to move the switch 20 in steps only upon one complete rotation of the variometer 9. In other words, upon a complete change of the variable inductance in the circuit from its maximum to its minimum value is the switch 20 positioned, whereupon the variable inductance offers tuning of the selected band until it reaches its maximum value. At this point the reversal to minimum may again actuate the switch 2!) to its next succeding, or preceding position. Several modifications may well be made here and it is considered to be a matter of design within the skill of one practicing the art and without departing from the scope of this invention.

I claim as my invention:

1. A high frequency coupling system comprising a primary circuit including a self inductance connected between input terminals; a secondary circuit coupled thereto including a fixed inductance and a variable inductance effectively in series between output terminals and a fixed capacity, switching means selectively operable for rendering said variable inductance and said fixed capacity ineffective at one position thereof and conjointly effective at another position.

2. A high frequency coupling system comprising a primary inductance between input terminals; a secondary inductance coupled thereto and serially connected with a variable inductance between output terminals, a fixed capacity and switching means selectively operable, having intermediate and oppositely extreme positions whereby said variable inductance and said fixed capacity are effectively disconnected at one of said extreme positions and said fixed capacity and said variable inductance together with said secondary inductance forming a series circuit are bridged between output terminals in the other of said extreme positions.

3. A high frequency coupling circuit adapted to operate over a wide frequency range comprising a primary circuit including a primary coil; a secondary circuit coupled thereto including a secondary coil and an inductance of progressively varying electrical characteristics, a fixed capacity and means for progressively connecting said inductance and said capacity to render said secondary circuit progressively resonant throughout said band.

4. A high frequency coupling system tunable over a wide range of frequencies comprising a primary circuit including a primary coil between input terminals; a secondary circuit inductively coupled thereto by means of a fixed inductance; a progressively variable inductance and a fixed capacity connected between output terminals of said secondary circuit, said variable inductance being in series with said secondary coil and means automatically operable upon the actuation of said variable inductance and at definite points of inductance values thereof for additively correlating the efiect of said variable inductance and said fixed capacity wlfereby the ratio of inductive reactance to capacitive reactance is maintained substantially uniform over said tunable range.

5. A high frequency coupling system tunable over a wide range of frequencies comprising a primary circuit including a primary coil between input terminals; a secondary circuit inductively coupled thereto by means of a fixed inductance; a progressively variable inductance and a fixed capacity connected between output terminals of said secondary circuit, said variable inductance being in series with said secondary coil and means automatically operable upon the actuation of said variable inductance and at definite points of inductance values thereof for additively correlating the effect of said variable inductance and said fixed capacity when tuning toward the lower frequency end of said band and subtractively correlating said effects when tuning to the higher frequency end of said band.

THEODORE P. KINN. 

